A foaming agent used for thermal-insulating polyurethane plastics is characterized by containing HFC-365mfc. When used as a foaming agent, the foaming agent comprises ingredients in parts by weight as follows: 3-40 parts of HFC-365mfc, 0.3-3 parts of water, 1-5 parts of catalysis assistant, and 45-85 parts of polyether or polyester. By adjusting the parts of HFC-365mfc, the density of free foaming can be effectively adjusted. Combined polyether can be prepared by mixing and uniformly stirring polyether or polyester, catalysis assistant and water and then adding three parts of HFC-365mfc foaming agent in (1) at the temperature of 40 DEG C to be uniformly mixed with the three materials above. In order to reduce CO2 generated in foaming reaction and heat conduction coefficient, the water content in the formula can be selectively decreased, and the prepared combined polyether and isocyanate are mixed according to a certain ratio to react and consequently produce polyurethane foam containing HFC-365mfc. The foaming foam has the advantages of low heat conduction coefficient, good foaming mobility, high foam strength, strong adhesive force of base materials, low molding density and environment-friendly and energy-saving materials.